Humans function in an “analog” realm; that is, nature is analog. In nature, sound behaves like a sine wave; it has a frequency and amplitude. With respect to signals representing sound, “digital” is the attempt to approximate, to cut down on the complexity caused by the infinite number of states that nature can achieve. The gramophone-type vinyl records play back sound that was taken as a continuous signal, with an infinite number of values. So too with analog audio tape recordings. On the other hand, electronically readable media including optical disks such as CDs and DVDs; digital audio tape; and memory cards such as Secure Digital and Memory Stick, store digital representations of sound that has been sampled at discrete intervals, usually on the order of nanoseconds. CDs and the other such media are “digital” in that, at the lowest level, everything is represented as either a zero or a one. The time slices at which the samples are taken are so small that it is impossible to recognize when listening that a certain amount of information from the continuous “analog” realm of nature has been omitted. On the other hand, this zero-one (binary) implementation makes the signal representing the music much easier to manipulate.
In addition, one of the great advantages of a digital recording is that it is highly stable with regard to timing, since, unlike analog recordings which rely on electromechanical devices to maintain their speed and pitch accuracy, digital recordings are limited only by the resolution of the master clock oscillator driving all the computation, which typically is a quartz device with precision typically within parts per million.
In many instances, analog recordings are digitized, to create a second master recording in a digital format, either because in the most obvious case the original recording was analog, and a transfer to CD or DVD or other digital medium is desired. There are also those who are familiar with the mechanical razor blade editing of tape or film or who prefer the particular way the analog processing affects the sound of the recording. In any event at some point for mass commercial release it must be converted to a digital medium. The digital medium is used to make the optical disk (e.g. CD or DVD).
Recordings which are made with analog techniques are invariably subject to timing errors, such as wow and flutter (hereinafter sometimes “wow/flutter” for short), resulting from speed variations in tape travel and other errors which degrade the quality of the sound. Fluctuations of one or two per second are generally classified as “wow,” while faster variations are called “flutter.” Even longer term variations than “wow” are sometimes termed “drift” but would be included within the compass of “wow/flutter” as used herein.
Some workers in the field have sought to mitigate those variations and errors in various particular ways.
U.S. Pat. No. 4,535,368 to Schwartz and Wu, describes apparatus for correcting instantaneous frequency deviation in the output signal of a playback device. The apparatus described comprises means for providing a constant-frequency signal at the output of the playback device in addition to the normal recorded program material to be reproduced by the playback device. The apparatus further includes a frequency-to-voltage converter having an input coupled to the playback device, which receives the constant-frequency signal, and produces an output voltage proportional to the frequency of the input signal; and a voltage-averaging means receiving the frequency-to-voltage converter output, which averages the input signal over a predetermined time period and produces an output signal representing the constant-frequency signal as originally provided. The output of the frequency-to-voltage converter and the output of the voltage-averaging means are both fed to a means for comparing these outputs, which produces an output voltage proportional to the frequency of a control signal required to correct the instantaneous frequency deviations in the output of the playback device. The output of the aforesaid comparing means is fed to a voltage-to-frequency converter, which produced a first control signal. A delay device receives the output of the playback device. The delay device has a clock drive that is driven by a control signal of variable frequency and coupled to the output of first voltage-to-frequency converter. The intended result is that the signals representing the normal recorded program material as reproduced by the playback device are selectively delayed by a time proportional to the frequency of the first control signal to thereby substantially correct for the detected instantaneous frequency deviations in the playback device's output.
U.S. Pat. No. 5,748,397, to Yamada, describes an audio-signal-processing apparatus and recording and/or reproducing apparatus for a tape-shaped recording medium. The reproducing apparatus includes a recording and/or reproducing head, a detecting section, and a compensating section. The reproducing head records or reproduces from analog signals and a detecting section detects any fluctuation in the transporting speed with respect to a rated transporting speed. The compensating section modifies analog signals, read from or to be recorded on tape by the tape head, so as to compensate for any fluctuation in the transport speed of the tape during recording or reproduction. The fluctuation in the transport speed on the basis of detection results from the detecting section before these analog signals are output or supplied to the recording and/or reproducing head. The audio signal processing apparatus includes a clock pulse generating section, an A/D-converting section, a computing section, and a signal generating section. The A/D-converting section converts input analog signals into digital signals in accordance with clock pulses from the clock pulse generating section. The computing section computes any fluctuation in the transporting speed of a tape with respect to a reference transporting speed on the basis of the results of the detection of the actual transporting speed of the tape. The signal-generating section converts the digital signals from the A/D-converting section into analog signals in which any fluctuation in the transporting speed of the tape with respect to the reference transporting speed has been compensated for on the basis of detection results from the computing section, and outputs the analog signals thus generated.
The audio-signal-processing apparatus can alternatively include a computing section, and first and second sample-and-hold sections. The computing section computes any fluctuation in the transporting speed of a tape with respect to a reference transporting speed on the basis of the results of the detection of the actual transporting speed of the tape. The first sample-and-hold section performs a sample-and-hold operation in a first cycle on inputted analog signals. The second sample-and-hold section performs a sample-and-hold operation in a second cycle on the output from the first sample-and-hold section. The second cycle is produced to compensate the first cycle according to the computing result from the computing section. Input/output timing of analog signals is controlled to vary in accordance with the results of the detection of the transporting speed of the tape. This reduces wow/flutter due to fluctuations in the transporting speed of the tape from the reproducing signals or the signals recorded on the recording medium. It is noted that Yamada teaches a speed system which is reliant upon mechanical correction using a tachometer device and is limited by the resolution of the tachometer, as is the case of any mechanically based system. It is also unable to deal with any problems that arise due to improper storage of tape or other physical medium, such as tape stretch or edge ruffle.
Neither of the aforementioned systems is directed to correcting adverse effects on recordings made on other equipment. Although some recordings incorporate a pilot tone or other time markings laid down purposefully on a companion track on the tape at the moment of recording, e.g. pursuant to standards promulgated by the Society of Motion Picture and Television Engineers, and such as are incorporated in audio recorders available from Nagravision S A (Cheseau, Switzerland), a need exists for a method and apparatus to correct wow/flutter in existing audio recordings that do not incorporate purposeful time markings.
Many beloved recordings exist that sound quite good by today's standards, but all of them are afflicted to some extent with speed variations brought about by the mechanical device employed to move the medium at the moment of recording. It would be highly desirable if a method were available by which the higher timing accuracy of digital recording could be employed to somehow refurbish the older analog recordings such that they could maintain a technical footing in this era.